Black Swan

What we call here a Black Swan (and capitalize it) is an event with the following three attributes.

First, it is an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility. Second, it carries an extreme impact. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurrence after the fact, making it explainable and predictable.

The Black Swan: The Impact of the Highly Improbable, Nassim Taleb.

The Black Swan metaphor is usually applied to events on a national or global scale, but from my perspective, it also accurately describes the extreme precipitation and flooding event that occurred in the Ozarks (and beyond) on April 29-30 of this year. And though the event covered the entire region and the heaviest precipitation and flooding were actually just west and north of my location, I’m going to focus on the Current River drainage because, well, I’ve lived here for a very long time and it’s an area with which I’m intimately familiar.

Precipitation recorded at the USGS gage at Van Buren, April 25-May 2.

Things got going on April 26, when Van Buren received almost 3” of rain, saturating the soil and pushing the river to a crest of 10.84 feet (3-3.5 feet is considered “normal” ) at 2:30 a.m. on the 28th. By the morning of the 29th, the river had dropped back to 7.28 feet, but the pump was primed and a watery hell was about to break loose. During the day of the 29th and into the 30th, the USGS gaging station at Van Buren recorded an additional 8” of precipitation and the headwaters of the both the Current and Jacks Fork received even heavier amounts.

Total precipitation estimated from radar, satellite and ground reports for the 14-day period from (8:00 am) Friday, April 21, 2017, to Friday, May 5. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service

The river began rising rapidly, going from the aforementioned 7.28 feet to a crest of 37.2 feet at 7:30 p.m. on the 30th.

Gage height readings at the USGS gage in Van Buren, April 25-May 2.

It overflowed the MO-106 bridge at Powder Mill upstream and the MO-160 bridge at Doniphan downstream. In Van Buren, it reached the Brookstone Apartments and the 2nd deck of The Landing on the west bank.

The Landing, looking downstream from the Highway 60 bridge. You can't see the first level of the building.

The main part of town lies on the east bank and there it wreaked havoc. The sheriff’s office was completely inundated and the CenturyLink central office had 8.5’ in it, knocking out all phone and data service that passed through it.

The Carter County Sheriff's Office, First Midwest Bank and the old Mercantile Building as seen from Highway 60

The most iconic business in Van Buren is the Jolly Cone and it had what looked like 10’ of water in it. The Carter County courthouse had 8’. The Post Office, License Bureau, Float Stream and Las Margaritas restaurants, G&W Food, Riverways Discount, First Midwest Bank and McSpadden Funeral Home along with the Methodist, Baptist and Catholic churches all had significant water in them. Over 100 residential buildings (an unfeeling way of saying “people’s homes”) were flooded.

The Carter County Courthouse, G&W Food, and the First Methodist Church, also from Highway 60

To put that in numerical perspective, the highest historical crest at Van Buren was 29 feet on March 26, 1904 (according to local lore, this was a “forty-footer,”) though there was another flood on August 21st, 1915 that may have been higher. Neither were actually measured with a gage, but were “determined by Missouri State Highway Commission from several reliable high-water marks in vicinity of gage” (Searcy, 1955). The highest crest that was actually gaged was on November 13, 1993, and reached 27.39 feet.

So the 2017 flood was more that 8 feet higher that the official record and nearly 10 feet higher than the previously highest gaged flood. Those represent differences of 28% and 36% respectively. So this year’s flood didn’t just exceed the previous records, it utterly destroyed them.

And here’s where the Black Swan metaphor comes into play. I honestly didn’t believe that a flood of this magnitude was possible. I simply didn’t. I’m 50 years old and I’ve seen quite a few floods that exceeded 20 feet, including the 1993 flood and I had a hard time imagining the 1904 flood, much less this one.

Actual Current River floodplain, just upstream of Highway 60 bridge, produced in Google Earth. River levels are approximate but should be pretty darn close to accurate.If you were to produce this cross-section 1/4 mile further upstream, it would encompass both Walker Hollow (the little creek that flows through Van Buren) on the east and the massive Pike Creek valley on the west, increasing the width and volume of the floodplain exponentially.

Let me explain why. As the river gets higher, the floodplain gets wider and it takes much more water to raise it further. For example, during the 10.84-foot crest on the 28th, the river had a discharge of 17,400 f3/s. It would take an additional 74,600 f3/s to go from there to the 27.39-foot crest in 1993 which had a discharge of 92,000 f3/s. The (still provisional) discharge during the April 30 crest was 177,00 f3/s, 85,000 f3/s more than in 1993. So, to raise the river to a level 36% higher than the 1993 crest took 92% more water. In other words, it took almost as much water to go from 27’ to 37’ as it would go from 0’ to 27’ and while the 2017 flood was 10’ higher than in 1993, it was almost twice as large by volume.

Discharge at the USGS gage in Van Buren, April 25-May 2.

Think about that. That’s f*cking insane and the reason I couldn’t even conceive of a flood like this one. And at one point, the National Weather Service was forecasting a crest of 43’, so in a way, we were lucky to “only” have gotten 37’. It could have been worse.

So, was this a Black Swan event? We have accounted for the first two attributes—this flood was way outside the realm of regular expectations and it absolutely carried an enormous impact on the local communities. As for the third attribute, rationalizing an explanation after the fact, here’s my take.

We live in a warming world. we’re all used to seeing the headlines about the long string of warmest months on record, the historic lows in Arctic ice coverage and the like, but locally, there are indicators as well. Spring and autumn, defined by leaf out and leaf fall, are occurring two to three weeks earlier and later respectively, than 20 and certainly 30 years ago. We haven’t had winter, not really, since 2014-15 and those extremely mild winters are becoming the norm.

Percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events) from 1958 to 2012 for each region of the continental United States. Source: National Climate Assessment.

Our climate is changing, becoming warmer, whether you accept it or not. Warmer air can hold more moisture, which equals more precipitation and a larger percentage of that precipitation is falling during very heavy events. Computer models predict that a warmer climate leads to more frequent and more powerful extreme events and that certainly seems to be happening.

In the future, expect more frequent and extreme droughts, more powerful storms and tornadoes, and larger floods that occur more often. Most areas of the United States are projected to see an additional 2-4° rise in temperature over the next few decades, so it’s only going to get worse. What’s now a once in a lifetime event at best, may well become the new normal.

References:

“Billion-Dollar Cyclone: The Human Element in This Week’s U.S. Floods.” Weather Underground. May 5, 2017. Accessed May 18, 2017. https://www.wunderground.com/cat6/billion-dollar-cyclone-human-element-week%E2%80%99s-us-floods.

“USGS 07067000 Current River at Van Buren, MO.” USGS Surface Water for Missouri: Peak Streamflow. Accessed May 18, 2017. https://waterdata.usgs.gov/mo/nwis/peak/?site_no=07010000.

USGS Current Conditions for USGS 07067000 Current River at Van Buren, MO. Accessed May 18, 2017. https://waterdata.usgs.gov/mo/nwis/uv?cb_00045=on&cb_00060=on&cb_00065=on&format=gif_default&site_no=07067000&
period=&begin_date=2017-04-25&end_date=2017-05-02.